1. Field of the Invention
The invention relates to an abnormality detecting system for a battery assembly, and more particularly to a technology of detecting an increase in the internal resistance of any battery cell in the battery assembly having a plurality of battery cells connected in series.
2. Description of the Related Art
A battery assembly in which a multiplicity of battery cells or battery modules are connected in series has been generally used. The battery assembly of this type is used in a hybrid vehicle, or the like, as a power source for driving an electric motor.
In a battery assembly as described in Japanese Patent Application Publication No. 2005-345124 (JP-A-2005-345124), an A/D converter is shared among a plurality of battery modules. In particular, JP-A-2005-345124 discloses a data collection system having first A/D converters for detecting voltage values and a second A/D converter for detecting current values, which converters are shared among a plurality of battery modules. The data collection system is arranged to operate the first A/D converters and the second A/D converter in an asynchronous mode, so as to ensure synchronism between the voltage detection values and the current detection values.
With the above arrangement, the voltage and current of the battery cells or battery modules are detected in synchronization with each other, so that the internal resistance can be correctly detected even if the battery voltage varies with current. It is thus possible to reduce the possibility of erroneously detecting an abnormality (increase) of the internal resistance.
In order to detect overcharge and over discharge for each battery cell in the battery assembly, each battery cell needs to be provided with a voltage monitoring function. In particular, when the system is arranged to be able to obtain a voltage value (analog value) for each battery cell, an abnormality (excessive increase) in the internal resistance can be detected based on a combination of the voltage value obtained for each battery cell and a current value obtained by a current sensor. With this arrangement, the data collection system as described in JP-A-2005-345124 has the A/D converters for voltage detection, each of which is shared by the battery cells or battery modules, and is operable to synchronize the current detection values and the voltage detection values.
However, if the abnormality detecting system is arranged to be able to output a voltage detection value for each battery cell, the number of sensors installed in the system and the number of signals handled by the system are increased, which may result in a complicated arrangement or construction of the abnormality detecting system, and an increased size of the system.
Accordingly, the system may be arranged to monitor the voltages of the battery cells by generating only the result of comparison between the output voltage of each battery cell and a criteria voltage as a threshold value for determining overcharge or over discharge, rather than directly handling voltage detection values. This arrangement may simplify the construction or configuration of the system.
In the meantime, how to assure sufficiently high accuracy in abnormality detection, as well as simplification of the system, is an important matter. For example, since the battery voltage varies with current, the voltage detection timing and the current detection timing need to be in synchronization with each other, so as to accurately evaluate the internal resistance of the battery. Thus, if the abnormality detecting system is simply constructed as described above, it may be difficult to detect voltage values and current values while bringing them precisely into synchronization with each other. In this case, an abnormality (excessive increase) in the internal resistance may be erroneously detected, because of an error in current detection due to loss of synchronism.